there are also those early match specialties that accept (or even require) a prelim medicine year as well
the two that i can think of are neurology and opthomology
technically either a transitional year or a prelim year can fulfill the requirements for certification in neurology and optho. you'll find out in jan where you match for these two fields, hence you submit a prelim/transitional rank list at the end of feb knowing where you'll be for the rest of your training
that way you can choose to rank programs where you'd like to be (for example, most early matchers prefer to do their first year in the area where they went to med school and/or where they'll be doing the rest of their training so that they don't have to move twice, or they may choose to try for a tranny/prelim spot in a place that they'd like to live for a year)
it's nice cause you don't have to submit those regular match contingency lists (if i match at program X, my rank order for trannie/prelim is A, B, C; but if Y, then C, A, B....), but from my classmates that did it that way there didn't seem to be many problems.
just remeber to apply and interview for a good number of prelim (and trannie if acceptable for your field) spots as well as your chosen field so that you can have a good number of choices for both (i kindof screwed myself over by not having interviewed at enough prelim places cause i was sick of interviewing...)
and mpp is right, some program include the first year (or even require that you do it w/in their system), while others require you to match separately
and yes, the prelim year of internal med is different from a transitional year. prelim medicine is all adult internal medicine (+/-ER rotation), while a tranny involves rotating thru lots of diff fields (i.e. IM, Ob/Gyn, Peds, Surgery, ER ...)
